Fluid catalytic cracking (FCC) processes involve the interaction of a feedstock and a catalyst flowing through a reactor. Cracking of the feedstock occurs in the reactor at temperatures usually in the range of 890.degree. to 970.degree. F. resulting in a coke build-up on the fluid catalyst. The "spent" catalyst is processed to remove occluded oil and regenerated to be recycled to the process stream. The cracked oil is passed to a fractionator where gas, light oil and heavy oil products are separated, the heaviest and highest-boiling oils--fractionator bottoms--contain some catalyst fines and are routed back to the FCC reactor or to a settling tank for fines removal prior to use of this heavy oil in fuel oil or as carbon black feedstock.
The fractionator bottom oils containing catalyst fines are usually routed to a "rundown" tank where solid particles, mostly catalyst fines, are partially settled by gravity action, the top-most layer of oil in the tank is decanted off for product use. Near the bottom of the tank is a floating oil/solid mixture which is substantially impossible to separate by gravity settling. Eventually there builds up a large sticky mass of heavy fluid-coated floating oil/solids which accumulate in the rundown tank. It then becomes necessary to clean out the tank bottom and dispose of this waste. Rundown tanks commonly in use are as large as 100 to 150 feet in diameter, 30 to 50 feet in height, and can contain up to 10 feet of floating oil/solids and other sediments in the bottom.
Past practice has been to place personnel and equipment inside the tank and physically remove the floating oil/solid sediments to exterior equipment. The solid materials have been transported to "landfarming", landfills, or other treatment-disposal. The "landfarming" process consists of controlled application and cultivation of wastes on soil, on a properly engineered site, in order to use microbes naturally present in the soil to decompose the organic fraction of the wastes.
The past practice process has been extremely time-consuming both in physically removing the sediments from the rundown tank and in biological decomposition of the large volumes of oil containing high-boiling polynuclear aromatics. Further, land-farming or landfill areas are becoming scarce, and are frequently separated some distances from the refinery location. A more efficient and environmentally safe cleaning process has therefore been needed.
The present invention is of increasing importance because of recent toxic laws and OSHA restrictions on all aspects of the petroleum industry. FCC fractionator bottoms are recognized as containing polynuclear aromatics. Because the method of present invention can be accomplished totally from outside of the tank involved, the method may be considered as a substantially environmentally safe process.
While the foregoing has referred specifically to an FCC process, it should be understood that the invention applies to any process where a rundown tank collecting sticky solids is likely to be encountered.